Author Topic: Filament voltage and current in and old valve rectifier  (Read 858 times)

Offline Alberto

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Filament voltage and current in and old valve rectifier
« on: May 24, 2023, 06:39:23 PM »
Hello.

I got this nice old valve rectifier



In the glass is wirtten ATES 8008   made in Italy RCA license.

I have looked up in the internet and the filament values are 5V and 6,75 amps. Before triying it I measure the resistance of the filament and is around 0,1 ohms, and of course, when I connect it, when the voltage is 0,35V the current goes to 1,3 Amps and raising faaster than the voltage, so I don´t know how to turn on the filament.

Any advice?

Thank you!

Best regards

Offline DashApple

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Re: Filament voltage and current in and old valve rectifier
« Reply #1 on: May 24, 2023, 07:23:01 PM »

The cold resistance of the tube will always be lower than its hot running resistance and due to this when you connect this to a voltage source you will experiance inrush.

If you set the voltage to 0.35V on a constant voltage source and wait you will see the current start to drop as the heater heats up and its resistance increases so you could manually do this over a period of time. The other option is if you connect it directly to a 5V  supply is to soft start it but placing a higher value resistor in series with the tube heater for 10 seconds or so then shorting this out with a relay or such, this limits the current to something you can pick, somehere around the expect hot heater resistance would be a good start.

In this case with 5V @ 6.75A the heater resistance is around 0.74 ohms when hot, something like a 1 Ohm soft start resistor would do but bear in mind it will be dissipating 25W or so at startup. There are also solid state soft starts based on MOSFETs on the web too, Though a relay / resistor and switch is an option as above.

Offline Alberto

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Re: Filament voltage and current in and old valve rectifier
« Reply #2 on: May 24, 2023, 07:53:49 PM »
Thank you.

I have tried rising the voltage slowly to maintain the filament at 6,25 amps and it took almost 10 minutes to increase the reistance. I give up when the voltage was around  3,6 volts.

But yes, it works and I really want to know if it work.

Offline Twospoons

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Re: Filament voltage and current in and old valve rectifier
« Reply #3 on: May 25, 2023, 03:08:58 AM »
Exceeding rated current at power-on is normal for a filament. The  6A @5V  spec is for steady-state, not an absolute limit.  If you really want to soft-start your filament you could limit the current to about 12A, with a voltage limit set to 5V.

Offline Alberto

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Re: Filament voltage and current in and old valve rectifier
« Reply #4 on: May 25, 2023, 10:02:31 AM »

Thank you!

And other question, is rated to 10Kv and 1,25 amps. But, with voltage I have to apply in anothe and cathode to be able to measure some corrent?
« Last Edit: May 25, 2023, 09:58:35 PM by Alberto »

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Re: Filament voltage and current in and old valve rectifier
« Reply #4 on: May 25, 2023, 10:02:31 AM »

 


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